1. Field of the Invention
The present invention relates to communications networks, and more specifically, to retransmission protocols in a multicast environment.
2. Description of the Related Art
There are three primary addressing mechanisms used in communications systems today. These addressing mechanisms are unicasting, multicasting, and broadcasting. Unicasting involves using an address that uniquely identifies a single sink as the destination for a data communication within a communications system. Broadcasting involves using an address that identifies all sinks in the system as destinations for a data communication. Finally, multicasting involves using an address that identifies a group of sinks (typically more than one and less than all of the sinks) in the system as destinations for a data communication.
One advantage common to multicasting and broadcasting is that, with multicast and broadcast addressing, a communication can be sent just once, and yet be received by multiple sinks, thus conserving transmission bandwidth relative to a unicast scheme, where the same communication would need to be sent multiple times to be received by multiple sinks.
In some communication systems (typically referred to as connection-oriented communication systems), a communication from a source to a sink can be retransmitted in the event of lost data (e.g., due to transmission failures that result from, for example, interference or framing errors). In such systems, when unicast addressing is used, as described above, a source sends a communication to a single destination. If the sink receives the communication without error, then the sink acknowledges the receipt of the communication back to the source by sending an acknowledgement message to the source. A sink can detect missing data from an error in a communications sequence numbering, for example, and then send a negative acknowledgement back to the sink. If the source fails to receive an acknowledgement or if the source receives a negative acknowledgmnent, then it will assume the communication failed and the source will typically retransmit the communication to the sink.
For multicast and broadcast addressing, it is less common to use an acknowledgement-and-retransmission scheme due to the amount of traffic that can be generated from acknowledgements and retransmissions when there are a large number of sinks and/or one or more noisy channels in the system. However, in some cases, where the number of multicast recipients is relatively small, an acknowledgement-and-retransmission scheme is desirable as it can provide some degree of error tolerance in noisy environments.
In such situations, however, a problem can arise if one or more of the sinks experience a significantly higher retransmission requirement than the others. This can be due, for example, to some sinks being at a greater distance from the source than are other sinks in a wireless environment, or one or more sinks being on a portion of a network that is exposed to more interference than other portions of the network. In these situations, the one or more sinks that require retransmissions can load down the source and/or network with retransmission requests, affecting the other sinks in the network that would not, otherwise, experience difficulty receiving communications from the source.
Accordingly, there exists a need for retransmission protocols for multicast and broadcast communications network environments that improve transmission efficiency and/or perceived quality of service in the communications networks.